Hmmmm, seems you have covered it all. Strange that it was running OK and then it heated up on you.
You replaced the 7lb radiator cap with a 15 lb, right? This is based on my 1968 Service Manual - 1968 and later used the crossflow type radiators. It also says it will raise the boiling point to 258 degrees at sea level. Keep in mind that the radiator should not be filled to the top as the antifreeze needs space to expand when hot. Again, Service manual says 3" below filler neck when cold, 1" below when hot.
My thoughts are the thermostat because it is pushing back out the radiator. I have had nothing but bad luck with these on my '97 Toyota. They work for a couple weeks, then won't open. Been through 3 of them. Finally just pulled the thermostat out, removed the center section by cutting the lower tabs that hold it in and re-installed the metal ring, less the guts, to act as a restrictor plate and support the O-ring type gasket. Runs really cool at moving speeds with air going through the radiator & will warm up to "operating" temp in local stop/go traffic.
That said, I would get a cheapy thermostat from the local parts store, cut the center element out, and install just the metal outer ring to see if that makes a difference. If it does, then you know where your problem lies.
It could still be the fan clutch, but you have a new one coming, so one more elimination. Do you have the 19 1/2 diameter 7-bladed fan? Again, 1968 Service Manual.
There was one poster here that had a lean condition that was causing overheating. His engine was built, and it needed more gas to feed it. Q-jet initial setting is to screw in the idle mixture screws, gently, until it bottoms out. Then screw out 6 turns. Screw in to lean the mixture out, but don't do this as you want to run rich to see if it makes any difference. I don't know how many turns it takes to get the screw completely out, but I might go 7 turns if it will take it.
You also want to make sure you have a good clean air filter as one that is dirty or restrictive will cut airflow down and cause the engine to run lean. I prefer and open element air cleaner/filter and as big as I can fit.
Idle speed with TH-400 in "D", A/C off is 500 RPM when
fast idle electric solenoid is not energized and 650 RPM's when it is. If you don't have the solenoid, then 650RPM's is what you want to shoot for (manual trans is higher @ 850 RPM's). Sometimes idle speed needs to be higher with a performance cam. Do you have the electric fast idle solenoid on your carb?
Timing is 9 degrees BTDC with vacuum hose disconnected to distributor & plugged -for stock engines. This can vary based on cam. What you want to shoot for is
Total Timing which seems to be around 30-34 degrees for a Pontiac @ 2300-2500 RPM's It is also important to have the correct vacuum advance in the distributor in relation to your engine.
I found this tip on the internet that might also help you, "I prefer direct manifold vacuum over a ported source as found on most carburetors. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing. One night I was at the drags and my car was running hot in the staging lanes, I swapped the vacuum advance from ported to manifold then it would idle all night at 175°. Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions. If you're like most hotrodders that is of no concern to you. If you have a big cam with a choppy idle then a vacuum advance hooked to manifold vacuum can really help. It will idle smoother and requires less throttle to maintain speed. Often a big cam requires you to open the throttle so far that the curb idle adjustment needles won't work. Hooking the vacuum advance to manifold vacuum will allow you to close the throttle some which may be enough for the idle mixture screws to work."
Attached is the
Secret & Restricted specs never before released to the general public for fear the Chebby guys would get a hold of it- yes folks, info like this comes right out of the Pontiac Service Manual which each and every one of you should have either for yourself or the guy who works on your car. Note that these specs are given per the
distributor model number based on engine application and model of car. Don't know which each fits, but we can use this as an example for all. And, just to throw off the Chebby guys, timing is listed by
distributor degrees which is 1/2 of crankshaft degrees, ie centifugal advance distributor degrees @ 13-15 degrees is 26-30 @ crankshaft degrees on your balancer.
OK, we will use the second block, dist
#1110431 as our example Lets begin with the factory setting of 9 degrees BTDC initial crankshaft timing found on your balancer -
Idle Timing. To that we add the Centifugal/
Mechanical Advance, 12 distributor degrees/24 crankshaft degrees @ 2500 RPM's for a
Total Advance timing of 33 degrees (9+24=33) @ 2500 RPM's as found on your balancer. It is easy to see that if you advance your timing 3 degrees to 12 degrees BTDC, you will affect your total timing the same amount for a Total Advance of 36 degrees.
Now if you wanted to go 14 degrees initial timing, our total timing would be 38 degrees. This might be way too much @ 2500 and we could experience detonation. The mechanical advance/weights in the distributor could be made so they do not advance to give us any more than 34 degrees total advance. This is where fine tuning comes in and a little mechanical know how helps. The rate of how quick the distributor weights advance the timing (timing curve) and at what RPM this happens can be adjusted with assorted springs - light, heavy, or a combination of each. Again, some trial and error required here as your car/engine combo might like its total advance to be reached at 3000 RPM's instead of 2500 RPM's. Use the factory settings to get you going and then adjust from thee if you feel you need to. :thumbsup: